This invention relates to plural-wavelength infrared detector devices comprising groups of detector elements formed in two or more levels on a substrate, the detector elements of one level having a different infrared response from those of the other level(s). The detector elements may be photoconductive or photovoltaic and may be formed in, for example, cadmium mercury telluride. More information about the wavelength characteristics of an object or a scene can be obtained by viewing it using such a plural-wavelength infrared detector device so that for example, such a device can permit the temperature of a radiation source to be measured independent of the emissivity of the source and the transmission of the intervening space.
Infrared detector elements whose material (for example a specific composition of cadmium mercury telluride) is optimized for detecting radiation upto one cut-off wavelength (for example 11 micrometers) may be used to detect radiation upto a lower wavelength (for example 5 or 8 micrometers), and so it is possible to form all the detector elements of some plural-wavelength detector devices from a single layer of the same material and to filter or otherwise separate the wavelengths incident on the detector elements. However, a more efficient plural-wavelength detector device is obtained using two or more different materials optimized to have different wavelength responses for the detector elements. This is most readily achieved by providing the different materials as different levels on a substrate.
Published European patent application EP-A-0 087 842 which corresponds to U.S. Pat. Nos. 4,555,720 and 4,625,389 discloses such a plural-wavelength infrared detector device comprising a group of infrared detector elements formed in a lower level of material on a substrate, and a group of upper-level detector elements mounted on the lower level and having an infrared response different from the lower-level detector elements. Electrical connections are provided from the detector elements to the substrate. The detector elements may be photoconductive or photovoltaic. The substrate may be, for example an insulating support carrying conductor tracks, or the substrate may be, for example, a silicon integrated circuit for processing output signals from the detector elements. The whole contents of EP-A-0 087 842 are hereby incorporated herein as reference material.
The invention disclosed in EP-A-0 087 842 permits the electrical connections of each upper-level detector element to extend to the substrate via a portion (hereinafter termed "upper-connection portion") of the lower-level infrared-sensitive material. In the form disclosed in EP-A-0 087 842, the upper-connection portion of the lower level is separated from the lower-level detector elements by a gap, and this gap is bridged by the upper-level detector elements. This provides a simple yet reliable manner of providing connections to the upper level without degrading the performance of the detector elements in each level.
In the specific embodiment illustratd in the drawings of EP-A-0 087 842, a linear array of detector elements are formed in each level; in this case the upper-connection portions and the lower-level detector-element connections are located around the periphery of the lower level. Especially in the case of a 2-dimensional array of detector elements in each level, it may be desirable to locate at least some of the connections at areas inbetween the detector elements. However, the provision of connections to detector elements in two or more levels can occupy a considerable space between the detector elements which reduces the area available for detecting the infrared radiation in the active portions of each level. This problem is particularly acute for a plural-wavelength detector device having large numbers of detector elements in a 2-dimensional array in each level.